Various measured and estimated parameters utilized in the control of a motor vehicle powertrain must be assessed for rationality because erroneous parameter values can lead to degraded emission control, fuel economy and performance. One such parameter is the fluid temperature of a fluidic automatic transmission since the transmission shift points are scheduled as a function of the fluid temperature. Additionally, the transmission fluid temperature can be used as an enabling criterion for assessing the rationality of other transmission parameters. Accordingly, there is a need for reliably assessing the rationality of the measured transmission fluid temperature.
Ordinarily, the rationality of a measured fluid temperature can be assessed simply by monitoring the measured temperature for characteristic changes in value as the powertrain warms up. However, such techniques are not adequate to reliably assess the transmission fluid temperature because certain operating conditions can result in a false indication of non-rationality. For example, the engine may be subject to external heating by an engine block heater, or the vehicle may be garaged for a prolonged period at one ambient temperature and then driven into a very different ambient temperature. Accordingly, what is needed is a way of reliably assessing the rationality of a transmission fluid temperature measure under various operating conditions.